1. Field of the Invention
This invention relates to measuring instruments and more particularly to measuring instruments with digital display.
2. Prior Art
In conventional length measuring instruments of the mechanical type, a vernier scale or a dial indicator is generally used to measure the dimensions. These conventional length measuring instruments have however been defective in that there is a possiblity of misreading the instrument due to reading errors and human error due to the fact that the user of the measuring instrument reads the mechanically measured dimension with his eyes. Such conventional length measuring instruments have also been defective in that the fixed calibrations on the main scale makes it impossible to make relative measurements even when such a measurement is desired.
With the progression in the art of digital displays in various industrial fields, digital displays have been progressively applied to various types of measuring instruments. Caliper devices are not an exception and there exists calipers with a digital display.
In one prior art device, a so-called indutosyn is employed in which a main scale and an auxiliary scale provided with comb-like printed patterns are movable relative to each other and a change in the value of current in a printed patterns due to the relative movement of the main and auxiliary scales is measured as the dimension. Another caliper device of the prior art is wherein a so-called magnescale is employed in which magnetic calibrations are provided on a magnetic tape and a signal produced due to the movement of the scales relative to the magnetic tape is sensed to measure the dimension. In still another caliper device, a so-called linear encoder is employed in which light is directed toward an optical measuring means, for example a first member of relatively large length carrying calibrations provided on a material having a high index of reflection. The light reflected from the first member is received by a second member of relatively short length carrying similar calibrations and the first and second members are linearly moved relative to each other so that bright and dark patterns of light or the moire fringe produced by the reflected light is sensed to measure the dimension. In yet another caliper device a so-called rotary encoder is employed in which a rotatable first member is rotated relative to a rotatable second member and the bright and dark pattern of the light or the moire fringe thus produced is sense to measure the dimension. However, in any one of such prior art caliper devices which have a digital display, the digital display is generally disposed separate from the caliper body, the main scale or the slider, and is generally electrically connected to the later by a signal transmission cable.
Thus, these prior art caliper devices with digital display type cannot be conveniently used on site since the necessity of providing an electrical connection to the cable leads to the inefficient use of the caliper device and leads to an inconvenience in handling.
Among various caliper devices with a digital display type, there is included a special type which employs an optical linear encoder for dimension measurements. In this caliper device, a linear scale of relatively large length is embedded in the body of a main scale and light is directed towards the linear scale from above or below so that the light reflected therefrom is received by an associated scale to measure the dimension. However, this caliper device utilizing reflected light is disadvantageous in that emitting element of large capacity or size is required in order to compensate for losses of light due to, for example, scattering and measurement with high accuracy is impossible due to difficulty of providing calibrations spaced apart from each other by a very small distance, for example 10 micrometers. In this type of caliper device, any attempt to improve the accuracy of measurement results inevitably in a very high cost.